Many different methods for the preparation of epoxides have been developed. One such method involves the use of certain titanium silicalite materials to catalyze olefin oxidation by hydrogen peroxide. This method is described, for example, in U.S. Pat. No. 4,833,260, which discloses a procedure (Example 35) wherein propylene is converted to propylene oxide. An isopropanol/water mixture is reacted with oxygen at 135.degree. C. to afford a mixture containing hydrogen peroxide. The mixture is thereafter used directly in a titanium silicalite-catalyzed epoxidation of propylene without intervening treatment or fractionation.
U.S. Pat. No. 5,384,418 (corresponding to application Ser. No. 08/241,215, filed May 10, 1994) describes an integrated process for epoxide production which also employs hydrogen peroxide derived from isopropanol oxidation in a titanium silicalite-catalyzed epoxidation, but teaches that removal of substantially all of the acetone from the isopropanol oxidant prior to use in epoxidation is advantageous. The patent additionally suggests that isopropanol derived from hydrogenation of the removed acetone could be employed to dilute the isopropanol oxidant to achieve the desired H.sub.2 O.sub.2 concentration within the epoxidation reactor. Under certain conditions, it is desirable to maintain relatively dilute (i.e., 1-10 weight %) maximum hydrogen peroxide concentrations during epoxidation since higher concentrations can result in poorer epoxide selectivity.
We have now unexpectedly discovered that a crude alcohol stream obtained by removing epoxide from the reaction product mixture exiting the epoxidation reactor can also be used for the purpose of diluting the hydrogen peroxide feed to the reactor. This result was surprising, since titanium silicalite-catalyzed epoxidations are known to be sensitive to the presence of trace impurities. U.S. Pat. No. 4,824,976, for example, indicates that the addition of certain basic substances to a titanium silicalite-catalyzed epoxidation mixture helps to minimize acid-catalyzed non-selective ring-opening reactions of the desired epoxide. As the aforementioned crude alcohol stream typically contains, in addition to isopropanol and water, measurable amounts of relatively heavy (high boiling) organic acids, glycols, and so forth, it would have been expected that the use of this crude alcohol stream to dilute the hydrogen peroxide feed to the epoxidation reactor would result in poorer yields of epoxide. Instead, we have now found that little or no loss of selectivity to epoxide takes place when such dilution is practiced as compared to the use of purified or refined isopropanol.